Coppersafe contains chelated copper and is used to treat fish with parasites and other conditions. There are both freshwater and saltwater versions of this. This copper treatment will kill most invertebrates, including octopuses. I doubt if you can remove all traces to make it ceph friendly, since cephs are very sensitive to copper.

unfortunately, the copper even seems to invade the silicone used to hold the panes of glass together in the tank, so unless you have an acrylic tank, it will probably never be able to be used for inverts...sorry!
Greg